Free piston internal combustion engines include one or more pistons which are reciprocally disposed within corresponding combustion cylinders. However, the pistons are not interconnected with each other through the use of a crankshaft. Rather, each piston is typically rigidly connected with a plunger rod which is used to provide some type of work output. For example, the plunger rod may be used to provide electrical power output by inducing an electrical current, or fluid power output such as pneumatic or hydraulic power output. In a free piston engine with a hydraulic output, the plunger is used to pump hydraulic fluid which can be used for a particular application. Typically, the housing which defines the combustion cylinder also defines a hydraulic cylinder in which the plunger is disposed and an intermediate compression cylinder between the combustion cylinder and the hydraulic cylinder. The combustion cylinder has the largest inside diameter; the compression cylinder has an inside diameter which is smaller than the combustion cylinder; and the hydraulic cylinder has an inside diameter which is still yet smaller than the compression cylinder. A compression head which is attached to and carried by the plunger at a location between the piston head and plunger head has an outside diameter which is just slightly smaller than the inside diameter of the compression cylinder. A high pressure hydraulic accumulator which is fluidly connected with the hydraulic cylinder is pressurized through the reciprocating movement of the plunger during operation of the free piston engine. An additional hydraulic accumulator is selectively interconnected with the area in the compression cylinder to exert a relatively high axial pressure against the compression head and thereby move the piston head toward the top dead center (TDC) position.
With conventional free piston engines, each piston is reciprocally disposed within a corresponding combustion cylinder, but is not rotated within the combustion cylinder. As the piston moves from a TDC position toward a bottom dead center (BDC) position, the piston head moves past and uncovers the exhaust outlet to allow the combustion products within the combustion chamber to flow therefrom. Since the piston head does not rotate within the combustion cylinder, the same portion of the piston head is continually disposed adjacent to the exhaust outlet. The portion of the piston head adjacent to the exhaust outlet has been found to have higher temperatures when compared with other portions of the piston head (e.g., when compared with the portion of the piston head adjacent to the combustion area inlet associated with the air scavenging channel). These thermal gradients and distortions of the piston head may cause thermal fatigue of the piston head over time, resulting in a decreased life expectancy of the piston head.
The present invention is directed to overcoming one or more of the problems as set forth above.